J. Doxey I/II, K. Ansdell II and M. Boulfiza III · 2019-01-10 · Microscopical Examination and...
Transcript of J. Doxey I/II, K. Ansdell II and M. Boulfiza III · 2019-01-10 · Microscopical Examination and...
Sulfate Attack
Sulfate attack can produce profound damage to cement
paste and is among the most destructive processes
effecting concrete. When sulfate is allowed to react with the
hydrated aluminate phases within concrete paste the
expansive mineral ettringite is formed (Eq. 1). As ettringite
forms within the paste it causes internal stresses which
result in crack formation (Figure 4). The additional calcium
required to produce ettringite is supplied by calcium
hydroxide in the paste[5]. Sources of sulfate are generally
external to concrete and are supplied by seawater,
groundwater or available soil moisture; however, internal
sources of sulfate are not uncommon. Excessive addition of
gypsum or sulfate-rich aggregate are common sources of
internal sulfate contamination[5] (Figure 5).
Ca4Al2SO4 · 12H2O + 2Ca2+ + 2SO42− + 20H2O → Ca3Al2O3 · 3𝐶𝑎𝑆𝑂4 · 32H2O (1)
(Aluminate) (Ettringite)
Figure 4: The plane polarized light image (left) shows acicular needles of
ettringite formed within an air void. The scanning electron microscope image
(right) shows cracking caused by fibrous to acicular crystals of ettringite growing
at the interface between aggregate (center - top) and within the surrounding
cement paste (bottom of image). (Images courtesy of Saskatchewan Research
Council).
Figure 5: Although gypsum is added to cement mixes to control the rapid
hydration or “setting” of the aluminate clinker phases, internal sulfate
contamination is more likely to be the result of sulfate-rich aggregates. The
above image shows gypsum replacement in microfossils contained within a
micritic limestone aggregate. Petrographic pre-screening of aggregates can help
identify these potentially deleterious minerals (Image courtesy of Saskatchewan
Research Council).
J. Doxey I/II, K. Ansdell II and M. Boulfiza III
I Saskatchewan Research Council, II Department of Geological Sciences, Department of Civil and Geological Engineering
Composite Rocks
Concrete is a manmade composite “rock” consisting of a
carefully proportioned mix of natural aggregate, water and a
cementing agent. The cementing agent used in the
manufacture of concrete is known as ordinary Portland-type
cement (OPC) and is colloquially referred to as “paste”. When
the tiny clinker minerals (Figure 1) contained in OPC are
reacted with water a powerful exothermic reaction takes place.
This reaction results in the formation of a host of hydrated
mineral phases (Table 1) that are responsible for the strength
that is nearly synonymous with the word concrete.
Table 1: Chemical composition of cement clinker minerals and their associated
hydration products [5]. Concrete’s strength is derived from calcium silicate hydrates.
Petrographic Images
Concrete is known to suffer from a variety of deleterious
reactions that may take place between individual aggregates
and concrete pore fluids, external contact with aggressive ions
in solution and even freeze-thaw cycles. These processes can
result in deterioration that ranges from subtle to profound and
typically result in significant loss of both strength and
durability (the engineering equivalent for “weathering”). The
following sections illustrate how the petrographic microscope,
the scanning electron microscope and x-ray microanalysis can
be used to confirm deleterious processes such as alkali
aggregate reactions and sulfate attack in concrete.
Preamble
“There is also a kind of powder from which natural causes produce
astonishing results. This substance lends strength to buildings. I shall
begin with the concrete flooring observing that great pains and the
utmost precaution must be taken to ensure its durability. On this, lay
the nucleus, consisting of pounded tile mixed with lime in the
proportions of three parts to one, and forming a layer not less than
six digits thick.”
– Vitruvius, 1st Century BC [7]
Background
Petrography has been used by geologists for over a century to
classify and describe naturally occurring rocks, but did you
know that it can also be used to describe concrete and
cementitious materials? In this context, the word
“petrography” is used in a synonymous fashion with the term
“petrology” and the cement industry has an expectation that
petrographic studies will be complimented with mineralogical
and geochemical data relevant to the aggregate and
cementitious materials used in its manufacture. This poster
attempts to provide an overview of concrete as a composite
“rock” and investigating the various ways in which petrography
and petrology may be applied in the study of these “other”
rocks.
Clinker Minerals
Figure 1: Ordinary Portland-type cement clinker minerals are used to
make cement. The tiny clinker minerals are formed by heating
crushed calcium, aluminum and silica-bearing rocks to approximately
1500C in a rotating kiln. The photomicrograph (left) primarily shows
strongly zoned crystals of the clinker mineral alite (Ca3SiO5). The right
hand image shows alite (blue), belite (brown), aluminate (colourless)
and lath-form ferrite. Field of view in both images is approximately
200µm (Images are from Campbell, 1999 [2]).
Analogous to Stone
Concrete contains pore fluids within the matrix of the cement
paste (Table 2). These pore fluids may interact with aggregate
in the concrete in ways that are analogous to the rock-water
interactions observed between natural rocks and water.
Concrete is also frequently subjected to chemical and physical
weathering phenomenon in its service environment. For
example, the ingress of aggressive ions in solution can
influence the chemistry of cement paste in much the same
way that meteoric waters influence chemical weathering in
natural rocks and regolith. Physical weathering processes
which produce fractures or abrasion in natural rock are known
to have comparable effects on concrete. Comparisons such as
these are significant in that they suggest that the same
petrographic and petrologic techniques used to describe
natural rock can be readily applied to concrete.
Table 2: Concentration of ions present in pore fluids after 180 days (water to
cement ratio = 0.5) (modified after Taylor, 1997[5]).
Alkali Aggregate Reactions
Alkali aggregate reactions occur when certain rock types come
into contact with the highly alkaline (~ pH 13) pore fluids
within concrete. Alkali silica reactions (ASR) occur when
aggregates containing reactive forms of silica (e.g. opal,
chalcedony, micro- or crypto- crystalline quartz, etc.) interact
with the alkaline concrete pore fluids to produce potentially
damaging expansive alkali silica gel (Figure 2). Alkali
carbonate reactions (ACR) cause similar damage to that
associated with ACR and occur when alkaline pore fluids react
to breakdown dolomitic aggregate into calcite, the sodium
carbonate mineral trona and brucite. It is believed that the
mineral brucite may be responsible for the resulting expansive
damage caused by ACR (Figure 3). Both ASR and ACR are
known to cause varying degrees of internal stress and
degradation leading to structural cracking and loss of both
strength and durability.
Figure 2: Crack propagation caused by expansive ASR gel is confirmed by
petrographic or electron microscopy (images courtesy of ww.fhwa.dot.gov [6]).
Figure 3: Rounded aggregates of dolostone (left) have been partially dissolved
(right) upon contact with highly alkaline concrete pore fluids (image courtesy
of Saskatchewan Research Council).
The Pantheon – 200 AD
The Pantheon in Rome is the
largest unreinforced concrete
dome ever built. The art of
concrete was lost after the fall of
Rome and was not rediscovered
again until around 1678 [1].
www.ancient.eu/Pantheon/
Thaumasite form of Sulfate Attack
The thaumasite form of sulfate attack (TSA) is arguably the
most devastating of all forms of sulfate attack. Thaumasite
favors cool (< 20C), damp, alkaline (>pH 10.5) climates and
an abundance of silicate and carbonate ions[3,4]. When
environmental and chemical conditions are conducive and if
available aluminum from the aluminate phases (1) has been
depleted the formation of thaumasite (2) will be favored[3,4].
3Ca2+ + 2S𝑖32− +2CO3
2− + SO42− + 15H2O → 𝐶𝑎𝑆𝑖𝑂3 · 𝐶𝑎𝐶𝑂3 · 𝐶𝑎𝑆𝑂4 · 15H2O (2)
(Thaumasite)
Carbonate ions may be supplied to the system by way of
aggregates, limestone interground with cement, atmospheric
additions of carbon dioxide or groundwater. The required
silicate ions are derived from the calcium silicate hydrates
within the cement paste itself, resulting in a total loss of
strength. Thaumasite and ettringite have similar crystal habits
(Figure 7) and it is believed that the two minerals form a solid
solution[4]. If suspected, TSA can be confirmed by constructing
atomic ratio plots from data collected by x-ray microanalysis
(Figure 7).
Figure 7: Thaumasite may be intergrown with ettringite (left) and the two minerals
are believed to form a solid solution. Thaumasite can be inferred using atomic ratio
plots (right). Si/Ca ratios that are less than 0.48 suggest silica depletion in the
paste while S/Ca ratios will be effected by carbonate ion additions[3,4].
Ion in Solution Low Alkali Cement (mmol l-1) High Alkali Cement (mmol l-1)
Na+ 0.08 0.16
K+ 0.24 0.55
OH- 0.32 0.71
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
S/Ca
Si/Ca
Ettringite
Thaumasite
Gypsum
Concluding Remarks
The analogous comparisons between naturally occurring rocks and
“rocks” that we have made provide a sound basis for using petrography
and other petrologic techniques to understand their chemical, physical
and mineralogical properties. The petrographic microscope and the
scanning electron microscope are well suited to observing the delicate
mineralogical and microstructural characteristics found in concrete. The
petrologists understanding of mineralogy and geochemistry is
fundamental to understanding how concrete interacts with and is
effected by its service environment and tools such as x-ray
microanalysis are a useful companion for such studies.
References
[1] Ancient History Encyclopedia. 2015. Pantheon. [Online] Available at: http://www.ancient.eu/Pantheon/
[Accessed November 2015].
[2] Campbell, DH. (1999). Microscopical Examination and Interpretation of Portland Cement and Clinker, 2nd Ed.
Portland Cement Association.
[3] Freyburg, E., Berninger, A.M. (2003): Field experiences in concrete deterioration by thaumasite formation:
possibilities and problems in thaumasite analysis", Cement and Concrete Composites, 25, 1105-1110.
[4] Sibbick, R.G., Crammond, N.J, Metcalf, D. (2003): “The microscopical characterization of thaumasite”,
Cement and Concrete Composites, 25, pp. 831-837.
[5] Taylor, H. (1997). Cement Chemistry, 2nd edition. Heron Quay, London: Thomas Telford Publishing.
[6] US Department of Transportation Federal Highway Administration, 1025,:Concrete Pavements [Online]
Available at:: https://www.fhwa.dot.gov/pavement/concrete/pubs/hif09004/asr11.cfm. [Accessed
November 2015].
[7] Vitruvius. (1st Century BC). The Ten Books on Architecture. (M. H. Trans.) Harvard University Press, 1914.